Question 5 of 6: Coal Fired Boiler — heat absorption components, firing system and ash

Question 5: Coal Fired Boiler — heat absorption components, firing system and ash (15 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part (a) — identifying the components. The page 12 outline is a conventional two-pass radiant boiler drawn with dashed steam lines and solid water lines, and the six components can be identified unambiguously by following those lines rather than by their shape. The circle at the top left, connected to the roof header by a short water line and to a downcomer running the full height of the unit, is the boiler drum. The tall left-hand chamber below it, whose walls are water-wall tubing fed by that downcomer and in which the flame is drawn, is the furnace. Two pendant banks hang in the furnace outlet: the left one, whose inlet comes from the convection pass and whose outlet is the topmost dashed line leaving the drawing, is the secondary superheater; the right one, which is fed from outside the boiler and discharges as the second dashed line, is the reheater. In the back pass there are two horizontal banks. The upper one, supplied with saturated steam from the drum, is the primary superheater; the lower one, on the solid water line, taking feedwater in from the right and discharging up the rear wall and along the roof into the drum, is the economiser.

The identification is settled by connectivity, not by position. Saturated steam leaves the drum, goes to the primary superheater in the coolest part of the gas path, returns to the secondary superheater platens in the hottest, and leaves as main steam; the reheater is on a separate circuit that starts and ends outside the boiler; and the only bank on solid water lines is the economiser.

Part (b) — the combustion system and how the fuel reaches it. The burner openings shown on the furnace walls, the absence of a grate and the water-cooled hopper at the bottom identify this as a pulverised fuel (suspension) firing system, the standard arrangement for a utility boiler of this size. Coal is pulverised so finely that it burns in suspension in a fraction of a second, which is what allows a furnace of this height to complete combustion in the free volume rather than on a bed.

Raw coal is drawn from the bunker through a gravimetric feeder, which meters it by weight so the fuel-air ratio can be held during load changes, and drops into a mill. The mill grinds the coal until roughly 70 per cent of it passes a 75 micrometre (200 mesh) screen; a classifier at the mill outlet returns oversize particles for regrinding. Hot primary air, tempered to between about 70 and 90 degrees Celsius, is blown through the mill and does three jobs at once: it dries the coal, it sweeps the ground product through the classifier, and it conveys the pulverised fuel through the fuel-and-air pipes to the burners. Secondary air, the bulk of the combustion air, is delivered separately by the forced draught fan through the air heater into the windbox that surrounds the burners, where registers swirl it into the fuel stream to shape the flame. Firing may be tangential, with burners in the furnace corners creating a single rotating fireball on the furnace axis, or wall-fired with opposed rows; either way the flame fills the furnace volume and radiates to the water walls on all four sides.

Part (c) — where the ash goes and in what form. The incombustible mineral matter in the coal leaves the boiler by two routes. Roughly 15 to 20 per cent of it agglomerates in the flame, falls out of the gas stream and collects in the water-filled hopper at the bottom of the furnace as bottom ash: coarse, dark, partly fused clinker, quenched on contact with the water and broken up by a clinker grinder before a submerged scraper conveyor or a hydraulic sluice removes it to a pond or silo. The remaining 80 to 85 per cent is carried out of the furnace by the gas as fly ash, a fine light-grey powder of spherical glassy particles mostly below 50 micrometres. A little of it drops out in the hoppers under the convection banks and the air heater, but almost all of it is collected downstream of the air heater in an electrostatic precipitator or a fabric-filter baghouse, from which it is drawn dry by vacuum or pressure conveying to a silo. Dry fly ash is a valuable pozzolan and is sold into cement and concrete production; in Canada its use is covered by CSA A3001, which is the main reason modern stations collect it dry rather than sluicing it. Any economiser hopper catch is coarser and is normally handled with the bottom-ash system.

Part (d) — radiation and convection surfaces. The furnace water walls receive their heat almost entirely by radiation from the luminous flame and the hot gas, and the secondary superheater platens hanging in the furnace outlet are also predominantly radiant, with a convective component. Everything in the back pass — the primary superheater, the economiser and the air heater — is convective, taking heat from gas that has already given up most of its radiant energy. The reheater in the furnace outlet is mixed, radiant on the faces the flame can see and convective elsewhere. The distinction matters operationally: radiant surface absorbs less as load rises (the flame temperature changes little while the gas mass flow climbs), whereas convective surface absorbs more, and the two characteristics are combined deliberately so that steam temperature stays flat over the load range.

Part (e) — purpose and placement of each surface. The economiser is a feedwater heater placed in the coolest gas at the exit of the back pass. Its purpose is to recover heat that would otherwise go up the stack by raising the feedwater from about 280 degrees Celsius towards saturation, which lifts boiler efficiency by several points; it must be last in the gas path because it is the coldest surface in the boiler and only there can the gas be cooled furthest without falling below the acid dew point. Placing it anywhere hotter would waste high-grade heat on a low-grade duty and risk steaming in the tubes.

The superheaters raise the saturated steam leaving the drum to the final temperature of about 538 degrees Celsius, which increases cycle efficiency and, just as important, keeps the moisture in the last turbine stages within the erosion limit. They are split into two because no single bank can do the job safely: the primary superheater takes the steam through the first part of the rise in the moderate gas temperature of the convection pass, where tube metal temperatures stay low, and the secondary or finishing superheater completes the rise in the furnace outlet where the gas is hottest, so that the largest temperature difference is available exactly where the steam is hottest and hardest to heat. Splitting the duty also puts a convenient point between them for the attemperator that controls final steam temperature.

The reheater returns the steam exhausted from the high pressure turbine to a high temperature before it enters the intermediate pressure turbine. Its purpose is the same twofold one — more work per kilogram of steam and drier steam at the low pressure exhaust — and it is placed in the furnace outlet alongside the finishing superheater because it must achieve a large temperature rise in a stream at low pressure, where the specific volume is high, the mass velocity in the tubes is limited and therefore the heat transfer coefficient on the steam side is poor. Only the hottest gas available can drive that duty through a reasonable amount of surface, and the pressure drop must be kept small because every kilopascal lost in the reheater is work lost in the turbine.